The present invention relates to a device for measuring and adjusting preloading on bearings, i.e. for measuring and adjusting mounting and operating loads on bearings, which employs piezoelectric and magnetostrictive means to apply such preloading. As is well known, piezoelectricity is the phenomenon whereby some crystalline bodies, generically called piezoelectric crystals, are electrically polarized as a consequence of a mechanical deformation of elastic nature, or vice versa are elastically deformed if subjected to the action of an electrical field. This second effect, known as inverse piezoelectric effect or Lippmann effect, is the one used in the present invention. Similarly, for the same invention it is possible to make use of the magnetostriction phenomenon, consisting of the fact that the dimensions of a body vary when it is magnetized or when in any case its magnetization is varied. In the remainder of the description, materials subject to the piezoelectric effect are mentioned, but this is not to be taken as limiting.
The characteristics of these materials have found application, among other fields, also in the means for preloading bearings, where they replaced purely mechanical adjustment devices able to provide a variable stiffness for a bearing assembly. Examples of this kind are represented by machining on electrical mandrels or drill heads where the bearing assembly must have high stiffness and low speed of rotation, typical of roughing operations, and low stiffness for high speeds of rotation, as in finishing operations. As is well known, another factor is represented by thermal expansion which can lead, in the rolling areas, to operating overloads which impact on the working life of the bearings.
The U.S. Pat. No. 4,850,719 in the name of Moseley et al., for instance, describes a variable stiffness bearing which uses piezoelectric means to apply a variable preload force to the bearing. The piezoelectric means preferably used are piezoelectric wafers, positioned between two bearings around a shaft and alternatively connected to alternating voltage and to ground to allow the wafers to become distorted in a manner directly proportional to the potentials applied. The wafers are positioned consecutively side by side and, with their deformation due to a variation of the applied potential, they exert a longitudinal force, parallel to the axis of the bearings. Moseley""s device has no means for adjusting the preloading, and thus serves solely to adjust the stiffness of the bearing.
Also known, in any case, is a device for adjusting the preloading for a main shaft bearing from the publication of Japanese Patent No. 8-35220 in the name of Hitachi Seiki Co. Ltd. It comprises means, generally springs, for imparting a thrusting force in a single direction from a side of the outer race of a bearing, means for conferring a preloading, in general to the rolling support through the outer race of the bearing pressing the other side of the outer race in the opposite direction against the thrusting means. Moreover, a unit is provided for measuring the preloading applied to the outer race by the preloading conferring unit, and control means including means for storing the optimal preloading value for the speed of rotation of the main shaft, means for comparing the optimal preloading, extracted from the data storing means for values corresponding to the speed of rotation of the main shaft, with an effective preloading measured by the preloading measuring means.
The aforementioned known device describes a device for adjusting the preloading in which the thrusting means and the counteracting or elastic return means are distinct components of the device, positioned in respectively opposite parts of the bearing subjected to stiffening.
A device of this kind is particularly complex and not always applicable to a bearing on already operating shafts.
Also known is a device for adjusting the preloading for a bearing, disclosed in Japanese Patent No. 8-25106 in the name of Honda Giken Kogyo. The adjustment of the preloading in this case is obtained by means of pressure in the axial direction by means of a piezoelectric element positioned on the side of the bearing that sustains a rotating shaft. The device for adjusting the preloading comprises a sensor to measure the load acting on the outer race of the bearing and means for commanding a voltage applied to the piezoelectric element as a function of the measured load. The means for applying the preloading are represented by a cylinder whose piston applies a force on a slide applied against the outer race at the opposite side of the piezoelectric element. In the Honda device, it is possible to adjust the preloading on a pair of beatings thanks to respective sensors, but the Honda device comprises, like the Hitachi device, thrusting means that are distinct from the counteracting or elastic return means and positioned on opposite parts relative to the bearings. As in the Hitachi device, also in the Honda device the piezoelectric means for applying the preloading is formed by wafers positioned mutually side by side. They exert a direct action on the bearings, concentrated in some points of the outer race. The means for conferring the preloading are separated from the thrusting means and thus the whole preloading device is substantially divided into at least two parts located on the opposite sides of the bearings. In the Honda device the sensors are displacement sensors positioned in contact with the peripheral surface of the outer race of the bearing; they are able to allow a measurement of the load on the bearing only if it is assumed that the inner races remain fixed in position. Since this cannot be verified with certainty, the indeterminate nature of the measurement leads to an inadequate operation of the entire device.
The positioning of the sensors, located in proximity to the various bearings, and their number increase the complexity of the device, both in terms of construction and operation.
As stated previously, such complexity in known devices makes it impossible to introduce them on existing bearing assemblies without substantial modifications and heavy adaptations of the component parts.
The present invention therefore aims to overcome the drawbacks of the devices for adjusting the preloading known from the prior art.
An aim of the present invention is to obtain a device of the type mentioned above, in particular of the type using piezoelectric and magnetostrictive means, which has a mechanically compact structure allowing it to be installed on bearing assemblies replacing or being added to the external spacer, in existing or newly designed machines.
Another aim of the invention is to obtain a device that is able to measure the axial load on the bearings due to tightening during the assembly phase, to the working axial thrusts, to the centrifugal forces and the thermal expansions of the machine whereon the bearings are mounted.
Another aim of the invention is to obtain a device that is able to suit the stiffness of the bearing assemblies according to the type of machining operation (drilling, boring, milling, contouring ,etc.), to the type of tools employed, to the type of material to be machined or to the precision to be obtained on the finished piece.
A further aim of the invention is to obtain a device that allows to adjust the load thanks to a feedback control system between the current measurement and the predefined and stored load values according to the dimensions that cause the load itself to vary.
The invention, as it is characterized by the claims that follow, solves the problem of providing a device for measuring and adjusting preloading on bearings surrounded by a casing, of the type comprising means for applying a preloading, in the form of one or more piezoelectric or magnetostrictive elements, able to apply a thrust defining a preloading on the outer race of bearings, reaction means for applying a force counteractive the preloading force, sensor means for instantaneously measuring the load and feedback control system for adjusting said load, which, from a general point of view, is characterized in that said means for applying a preloading and said reaction means are positioned to constitute a single actuator organ, comprising two ring elements for distributing the preloading, opposite and coaxial to the shaft, a first ring and a second ring, whereof at least one is destined to act against the outer race of a bearing and containing, interposed between them, two or more piezoelectric or magnetostrictive means for applying the preloading and return means associated to connecting elements between said ring elements; to said actuator organ being operatively adjacent an annular loading cell connected with an external feedback control system for measuring and adjusting said preloading.